Method for calendering a board web

ABSTRACT

A long nip (N) shoe calender formed of a shoe roll ( 10 ) and a thermo roll ( 20 ) is used in calendering, with a nip dwell time which is over 10 ms, advantageously over 20 ms. The nip pressure is below 3 MPa, advantageously below 1 MPa, and a surface temperature of the thermo roll ( 20 ) is over 200° C., advantageously over 250° C. The board web (W) surface to be pressed against the thermo roll ( 20 ) is moisturized before the nip.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national stage application of InternationalApplication No. PCT/FI01/00378, filed Apr. 17, 2001, and claims priorityon Finnish Application No. 20000927 filed Apr. 18, 2000, the disclosuresof both of which applications are incorporated by reference herein.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to a method for calendering a board web.

A Yankee cylinder is generally used in the manufacture of sufficientlystiff board grades which are suitable for packages, for example, forbiscuit/cookie packages, cosmetic packages, etc. and one side of whichis additionally required to have good surface properties. After thetreatment with a Yankee cylinder, the board surface which has beenagainst the Yankee cylinder is subjected to surface treatment. After thesurface treatment, the board web is further subjected to finalcalendering, when needed. The web treated with a Yankee cylinder isgiven good surface properties, good bulk and stiffness and low shrinkageat the edges.

The most important quality properties of board produced by a Yankeecylinder before coating are in a range: bulk 1.4-1.6 dm³′/kg, Bendtsenroughness 50-250 ml/min and PPS-s10 roughness 3.5-7.5 μm.

One problem with the use of a Yankee cylinder is its runnability. TheYankee cylinder can be run only in a relatively narrow operating window.The web must be sufficiently moist when it arrives at the Yankeecylinder in order that it should properly adhere to the hot and smoothouter surface of the shell of the Yankee cylinder by the action ofadhesion. On the other hand, the web must not be too moist when itarrives at the Yankee cylinder in order that it shall have time to drysufficiently on the Yankee cylinder. If the web does not have time todry sufficiently on the surface of the Yankee cylinder, it cannot bedetached from the surface of the cylinder at the trailing end of thecylinder. Separation of the web from the hot outer surface of the Yankeecylinder takes place by means of a doctor. The above-mentionedrunnability problems have limited the running speed of the Yankeecylinder with board grades of this kind to a range of below about 600m/min. Typically, the running speeds of the Yankee cylinder are in arange of about 200-400 m/min. Moreover, the shell of the Yankeecylinder, the diameter of which may be as large as 7 m, shall meetstrict requirements in respect of deformation, thermal conductivity,wear and corrosion, with the result that the Yankee cylinder will berelatively expensive. In addition, an impingement device is normallyused in connection with the Yankee cylinder.

The above-mentioned board grades can also be produced without a Yankeecylinder, in which connection the web can be provided with desiredsurface properties by means of a wet stack calender. The wet stackcalender is formed of a multinip hard nip calender, but the calenderingprocess totally differs from conventional hard nip calendering. The wetstack calender makes use of moisture gradients. The web is dried beforethe wet stack calender such that its moisture content is only about1-2%. On the wet stack calender, water boxes are used in connection with1-3 rolls for forming a film of water onto the outer surface of the rollshell before a nip. This water film is pressed onto the surface of theweb in the nip. The relatively thick web is moistened only from thesurface thereof, in which connection, by the action of simultaneouspressure, the web is calendered more on the surface as compared with theover-dried interior of the web. This kind of calendering results in agood smoothness to bulk ratio, i.e. good smoothness is obtained,however, without losing too much bulk. Surface treatment and possiblyfinal calendering of the web are carried out after the wet stackcalendering.

Runnability problems are also associated with the wet stack calendering.If the pressure distribution in the nips provided with water boxes isnot sufficiently uniform, water can pass through the nip, forming apocket of water underneath the web. This causes web breaks at the nextnip. Since bulk is a critical factor with board grades, it must bepossible to operate the calender with an optimal nip pressure requiredby each board grade, which pressure is sufficiently uniform in theentire area of the nip allowing the use of water boxes. The wet stackcalender is designed so that the number of rolls can be varied thereinand deflection-compensated rolls are placed such that a sufficientlyuniform nip pressure is achieved in the nips provided with water boxes.In the wet stack calender, wrinkles are also readily formed in the web,in particular in a web having a low basis weight.

Metso Paper, Inc.'s U.S. Pat. No. 5,938,895 discloses one wet stackcalender with a water box positioned in connection with a profiling nipformed by a deflection-compensated roll placed in the middle of thecalender. U.S. Pat. No. 5,522,312 discloses a wet stack calender inwhich a metering device is used in connection with a water box forcontrolling the thickness and uniformity of a water film applied to acalender roll. The water is transferred from the calender roll to theweb at a nip. U.S. Pat. No. 5,607,553 discloses a wet stack calender inwhich the water boxes have been replaced by water spray devices forspraying water in the form of droplets to a reversing roll of thecalender. The water is transferred from the reversing roll to the web ata nip.

WO 99/67462 discloses a method for calendering paper or board whenmanufacturing coated grades of paper or board in two stages. In thefirst stage, i.e. the pre-calendering stage, the uncoated web iscalendered with a shoe calender having a shoe length of at least 50 mmafter which the web is coated. In the second stage the coated web iscalendered with a calender having a nip length of 50 mm at the most. Themaximum pressure in the pre-calendering nip is kept at 0 to 15 Mpa,preferably at 4 to 12 Mpa. The web is pre-calendered at a moisture andtemperature where at least the glass transition temperature of thematerial forming the surface part of the web has been reached. The webmay be pre-treated e.g. by steaming and/or wetting with water or acombination of pre-wetting and the use of a heated backing roll in theshoe calender can be used in order to reach the glass transitiontemperature. The dwell time of the web in the calender is 3 to 40 ms.

WO 96/26809 discloses a coated paperboard for formed articles, e.g.liquid packaging board or white top liner. The paperboard consists of afibermatrix in one, two or more layers and a coating and has adequatesurface gloss for each specific type of formed articles. The paperboardhas been calendered only after it has been coated with a heated calenderhaving a soft extended nip. The specification mentions that the calendertemperature is typically in the range of 140-250° C., but even highertemperatures are possible. The length of the nip in the so calledsupersoft calender which can be used in calendering the coated web issaid to be 40 to 60 mm. Although the main idea in this document is tocalender only the coated web, i.e. not the uncoated web it is said thatit is also possible to calender the uncoated web in addition to thecalendering of the coated web.

WO 00/03087 discloses a method for manufacturing surface-treatedprinting paper. The web is first dried down in the drying sectiontypically to a moisture of 2 to 4%, and it is wetted again typically toa moisture of 8 to 12% before surface treatment, e.g. calendering in amulti-nip calender. The wetting device is placed before the calender sothat the absorption time of the water is 0.2 to 2.0 s before the webreaches the calender. The amount of water used in the method is 0.1 to10 g/m² and the droplet size in the spray-moistener is 10 to 100μ.

U.S. Pat. No. 5,836,242 discloses a calendering system in a papermakingor board manufacturing process. The system comprises at least one pressnip and an endless calender belt comprising a core and a compressible,elastic material bonded to the core. The belt comprises at least twolayers having different hardnesses. The web side has a first hardness inthe range of 75 to 91 Shore A and the layer on the press side has asecond higher hardness. The average pressure used in this calender ishigher than 4 MPa.

Because of the above-mentioned limitations, in the case of theabove-mentioned board grades it would be desirable to replace the Yankeecylinder and the wet stack calender with some new improved arrangement.As one new arrangement, trials have been carried out with the long nipcalender known in itself, but so far the results have not been goodenough.

By a long nip calender is meant a calender in which a nip is formedbetween a heatable steel roll and a belt. In the long nip calender, thenip pressure can be adjustable in the cross direction (CD) of themachine, in which connection it is possible to profile, for example, thecaliper of paper. The long nip calender can be formed of a belt calenderin which a belt is passed as guided by auxiliary rolls around one of thenip rolls, i.e. the roll operating as a backing roll for a thermo roll.Thus, a long nip is formed between a thermo roll and the other nip rollloading the belt. The most common long nip calender is a shoe calenderin which a belt is arranged to run around a stationary support structureand in which the belt is loaded against a thermo roll by means of aloading shoe positioned inside the loop of the belt and supported on thesupport structure. The long nip is formed in the shoe calender betweenthe thermo roll and the shoe loading the belt. Thus, the length of thenip is determined by the loading shoe of the shoe calender. The methodaccording to the invention is primarily suitable for use in connectionwith the above-mentioned shoe calender.

In the prior art shoe calenders which are in use, the length of the nipis typically in a range of 50-70 mm, i.e. the dwell time of the web inthe nip is considerably less than 10 ms. The surface temperature of thethermo roll serving as the backing roll of the shoe roll is in a rangeof 80-200° C. and the maximum pressure of the nip is in a range of 5-10MPa. The hardness of the calender belt of the shoe calender is in arange of 80-100 ShA. Longer nips of about 270 mm have been used inpresses based on the shoe roll.

SUMMARY OF THE INVENTION

In the method according to the invention, good calendering results havebeen totally unexpectedly achieved by calendering the web with a shoecalender in a parameter range that is contrary to the presentpreconception of a person skilled in the art.

In trials carried out on a shoe calender, it was unexpectedly found thatby operating the shoe calender with parameter values that considerablydiffer from the parameter values known until now, a good calenderingresult was achieved in particular with the above-mentioned board grades.In the shoe calender, a nip was used which was considerably longer thannormal such that a nip dwell time of over 10 ms, advantageously over 20ms, was achieved. In addition, a very low nip pressure of below 3 MPa,advantageously below 1 MPa, and a higher than normal surface temperatureof over 200° C., advantageously over 250° C. in the thermo roll wereused. The web surface pressed against the thermo roll was also subjectedto water moisturizing before the shoe calender.

In the method according to the invention, it is also advantageous to usea calender belt having a hardness of below 100 ShA, preferably below 80ShA.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention is described in the following withreference to the accompanying FIGURE, to the details of which theinvention is not intended to be exclusively confined.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The FIGURE schematically shows a shoe calender to which the methodaccording to the invention can be applied. The shoe calender compriseshere a shoe roll 10 and a heatable backing roll 20, i.e. a thermo roll.The shoe roll 10 is formed of a stationary support structure 11 and abelt shell 12 rotating around it. The belt shell 12 is loaded againstthe thermo roll 20 by means of a loading shoe 13 provided inside thebelt shell 12 and supported on the support structure 11 by means of tworows of actuators 14 a, 14 b spaced from each other in the machinedirection. The actuators 14 a, 14 b are advantageously formed of acylinder-piston construction. The loading shoe 13 comprises lubricantfeed ducts 15, by which a lubricant can be fed into a lubricant pocket16 between the belt shell 12 and the frontal surface of the loading shoe13, from which pocket it forms a lubricant film between the loading shoe13 and the belt shell 12. A long nip N is formed between the belt shell12 shaped by the loading shoe 13 and the outer surface of the shell ofthe thermo roll 20.

A web W is passed into the long nip calender from the direction shown bythe arrow S. The web W surface to be pressed against the thermo roll 20is moisturized by means of a moisturizing device 30 before the web W ispassed into the long nip N. The moisture content of the web W before themoisturizing device 30 can be in a range of 1-20%, advantageously in arange of 1-10%. The moisturizing is accomplished by spraying atomizedwater onto the surface of the web in an amount of 1-20 g/m². Water issprayed onto the surface of the web W such that its time of actionbefore the nip is about 0.1-2 s. The aim here is that only the web Wsurface to be placed against the thermo roll 20 is moisturized. Whenneeded, surface active agents can also be mixed into the water used formoisturizing in order to assist water to penetrate into the surfacestructures of the web.

The length L of the long nip N has been chosen according to the runningspeed at each particular time such that the dwell time of the web W inthe long nip N is over 10 ms, advantageously over 20 ms. For example, ata running speed of 1500 m/min, the length of the nip shall be about 500mm in order to achieve a dwell time of 20 ms.

The loading shoe 13 is loaded so that the web W is subjected in the nipN to a compression pressure of below 3 MPa, advantageously a compressionpressure of below 1 MPa. The compression pressure naturally also has alower limit, the desired calendering effect not being achieved atpressures lower than this lower limit. According to present knowledge,this lower limit is approximately about 0.1 MPa.

The thermo roll 20 is heated so that the temperature of the outersurface of its shell is over 200° C., advantageously over 250° C. Thethermo roll 20 can be heated by circulating in the thermo roll a heatingmedium which is heated in a heating device provided outside the thermoroll. The thermo roll 20 can additionally be heated, for example, withan induction heater 40 placed in connection with the outer surface ofthe shell of the thermo roll 20 or inside the thermo roll. For example,water, steam or oil can be used as a heating medium.

In the trials carried out it was found that, for example, the web usedfor the manufacture of board grades known under the trade names WhiteLined Chipboard (WLC) and Folding Box Board (FBB) could be calendered bythe shoe calender provided with the above-mentioned parameter valuessuch that the quality of the calendered product was as good as orsuperior to the quality of corresponding board produced on a Yankeecylinder.

The most important quality properties of a web calendered by the methodaccording to the invention before coating are thus in at least thefollowing range: bulk 1.4-1.6 dm³/kg, Bendtsen roughness 50-250 ml/min,and PPS-s10 roughness 3.5-7.5 μm.

The use of a Yankee cylinder and, as its alternative, of a wet stackcalender can be replaced with the method according to the invention inthe manufacture of at least the board grades WLC and FBB of theabove-mentioned type. As a result of this, the efficiency of boardmanufacture can be increased considerably. The running speed of a Yankeecylinder is in a range of less than about 600 m/min and otherrunnability problems are associated with a wet stack calender. Bycontrast, considerably higher running speeds can be achieved with theshoe calender according to the invention, and its runnability is good.The method according to the invention can be applied to a shoe calenderso that running speeds of even over 2000 m/min are achieved. The shoecalender does not in itself set any technical limitations to thecalendering speed.

The claims are presented in the following and the details of theinvention may differ within the inventive idea defined by said claimsfrom the disclosure given above by way of example only.

1. A method for calendaring an uncoated board web for manufacture of board grades White Lined Chipboard (WLC) and Folding Box Board (FBB) in a long nip shoe calendar, the method comprising the steps of: passing the web at a first speed through a long nip shoe calender having a long nip formed between a shoe roll and a thermo roll, the long nip having a length such that the dwell time of the web within the nip is greater than 10 ms, and applying a nip pressure below 3 MPa, and wherein a surface temperature of the thermo roll is greater than 200° C., and wherein a calender belt having a hardness of below 100 ShA is used on the shoe roll; and moisturizing the board web surface to be pressed against the thermo roll before the nip by spraying atomized water onto said board web surface in an amount of 1-20 g/m² such that the time of action of the water before the nip is about 0.1-2 s.
 2. The method of claim 1 wherein the nip dwell time is greater than 20 ms.
 3. The method of claim 1 wherein the nip pressure is below 1 MPa.
 4. The method of claim 1 wherein the surface temperature of the thermo roll is greater than 250° C.
 5. The method of claim 1 wherein the hardness of the calender belt is below 80 ShA. 